Developments in zeolite ctalysis and hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks for producing C.sub.5 + gasoline, diesel fuel, etc. In addition to basic chemical reactions promoted by medium-pore zeolite catalysts, a number of discoveries have contributed to the development of new industrial processes. These are safe, environmentally acceptable processes for utilizing feedstocks that contain olefins. Conversion of C.sub.2 -C.sub.4 alkenes and alkanes to produce aromatics-rich liquid hydrocarbon products were found by Cattanach (U.S. Pat. Nos. 3,760,024 and 3,756,942) and Yan et al., (U.S. Pat. No. 3,845,150) to be effective processes using medium-pore zeolite catalysts. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al. have also contributed to the understanding of catalytic olefin upgrading techniques and improved processes as in U.S. Pat. Nos. 4,150,062; 4,211,640 and 4,227,992. Brinkmeyer et al. U.S. Pat. No. 4,229,602 teaches a dehydrocyclization process in which liquid and gas product streams are recycles to increase conversion to aromatics. The above-identified disclosures are incorporated herein by reference.
The product stream from a medium-pore zeolite catalyzed aromatization process contains hydrogen, uncoverted aliphatic feed, light aliphatic by-products, and aromatics. The C.sub.2 + aliphatics may be recycled and aromatized. On the other hand, however, hydrogen and methane are not converted in the aromatization reaction and in fact hydrogen has been found to promote undesirable side reactions. Thus it has been discovered that it is highly desirable to remove hydrogen from the recycle stream.
The present invention provides an unexpected improvement in aromatics yield by removing substantially all of the hydrogen and methane from the recycle stream. Further, loss of valuable C.sub.2 + aliphatics to fuel gas is essentially eliminated.
In particular, the invention increases conversion of aliphatics to aromatics by recycling both a C.sub.5 + aliphatic stream and a highly purified C.sub.2 -C.sub.4 aliphatic stream to the aromatization reactor. In a preferred embodiment, C.sub.2 -C.sub.4 aliphatics are separated from hydrogen and methane in a dephlegmator process.
Dephlegmators are known in the art. For example, U.S. Pat. No. 4,270,940 to Rowles et al., incorporated herein by reference, teaches a process for the enhanced recovery of ethane and ethylene from demethanizer overhead by subjecting the uncondensed vapor effluent from the main reflux condenser to further condensation and accompanying rectification in a dephlegmator and returning the liquid condensate from the dephlegmator to the demethanizer column.
U.S. Pat. No. 4,519,825 to Bernhard et al. discloses a process for separating C.sub.4 + hydrocarbons in high recovery and high purity from a light gas feedstream using a dephlegmator cycle. A mathematical model of dephlegmation processes is described in DiCave, Mazzarrotta and Sebastiani, "Mathematical Model for Process Design and Simulation of Dephlegmators (Partial Condensers for Binary Mixtures)", 65 CANADIAN JOURNAL OF CHEMICAL ENGINEERING 559 (Aug., 1987).
For the purpose of this disclosure, the term "dephlegmation" is defined as a separation process as described above in which a refrigeration cycle is used to control partial condensation in a distillation process with countercurrent flow of rising vapor and falling condensate.